The storage of electrical energy has taken on increasing significance in recent decades. Electrical energy is storable with the aid of batteries. Batteries convert chemical reaction energy into electrical energy. A distinction is made in this case between primary batteries and secondary batteries. Primary batteries are capable of functioning only once, while secondary batteries—which are also referred to as accumulators—are rechargeable. A battery (also referred to as a battery module) includes, in this case, one or several battery cells (electrochemical cells).
Lithium-containing electrodes including lithium ions (a so-called lithium-ion battery) or even metallic lithium (a so-called lithium-metal battery) are frequently utilized in an accumulator. These are distinguished by, inter alia, high specific energy, good thermal stability, and an extremely low self-discharge.
Lithium-containing battery cells include one positive electrode and one negative electrode. The positive and the negative electrodes each include a current collector, on which a positive and a negative active material, respectively, has been applied. The positive and the negative active materials are characterized, in particular, by the fact that they are capable of reversibly incorporating and giving off lithium ions.
The active material for the negative electrode of such a battery cell is, for example, amorphous silicon which may form intercalation compounds with lithium atoms. Carbon compounds such as, for example, graphite, or metallic lithium are also widely used as active material for negative electrodes. A lithium-containing metal oxide or a lithium-containing metal phosphate is generally used as active material for the positive electrode.
In addition, the battery cells generally include a separator which prevents contact between the electrodes, and an electrolyte which enables the transport of charge carriers between the electrodes.
U.S. Pat. App. Pub. No. 2015/0104691 A1 describes secondary batteries which include electrolyte layers which include non-aqueous electrolytic solutions and a polymer compound. The polymer compound is a graft copolymer including a block copolymer made up of vinylidene fluoride units and hexafluoropropylene units as a main chain, and side chains of different homopolymers and/or copolymers.
U.S. Pat. App. Pub. No. 2009/0030152 A1 describes the use of different polyvinylidene fluoride-containing copolymers as energy-storage materials in capacitors.
During the operation of the battery cell, i.e., during a discharging process, electrons flow from the negative electrode to the positive electrode in an external circuit. Within the battery cell, lithium ions migrate from the negative electrode to the positive electrode during the discharging process. In this case, the lithium ions are reversibly removed from the active material of the negative electrode, which is also referred to as delithiation. In a charging process of the battery cell, the lithium ions migrate from the positive electrode to the negative electrode. In this case, the lithium ions are reversibly incorporated into the active material of the negative electrode again, which is also referred to as lithiation.
The lithiation or delithiation of the negative electrode is associated with a change in volume and/or shape. During the charging of a battery cell, an increase in volume, possibly accompanied by a change in shape, takes place, for example, when an Li-metal anode is used. In this way, a completely charged battery cell may have a volume which is increased by approximately 15% as compared to the discharged battery cell. This regularly occurring change in volume and/or shape results in local spatial displacements, for example, of the current collectors, and in changes in the filling of the battery cell. This creates difficulties with regard to the design of a battery including a large number of battery cells. The use of foams or similar materials which could compensate for the change in volume and/or shape may possibly result in different pressures acting on the electrodes and the separator depending on whether the battery cell is in the charged state or the discharged state. This may result in a non-uniform performance of the battery cell over the service life. Likewise, the charging and discharging processes influence the volume and/or the shape of the active material of the positive electrode. This generally occurs to a lesser extent than in the case of the negative electrode.